A nonaqueous electrolyte battery in which a lithium metal, a lithium alloy, a lithium compound or a carbonaceous material is used for a negative electrode is expected as a high energy density battery, and active research and development have been conducted. A lithium ion secondary battery including a positive electrode containing LiCoO2 or LiMn2O4 as an active material and a negative electrode containing a carbonaceous material that allows lithium ions to be inserted in and extracted from has been widely put to practical use for a portable device.
In the case of installing the battery in a vehicle such as an automobile or a train, it is desirable that the positive and negative electrodes include a material excellent in chemical and electrochemical stability, in durability, and in corrosion resistance for obtaining a storage performance in high-temperature environments (e.g., at not less than 60° C.), cycle performance, and reliability of high power over a long time. Further, high performance in cold climates, high-output performance in a low-temperature environment (−40° C.), and long life performance are required. On the other hand, although a nonvolatile and noncombustible electrolytic solution has been developed as a nonaqueous electrolyte for enhancing safety performance, a battery including the electrolytic solution has not yet been put to practical use because output characteristics, low-temperature performance, and long life performance are reduced.
As described above, when the lithium ion secondary battery is installed in a vehicle or the like, there is a problem with the high-temperature durability and low-temperature output performance. Thus, it is difficult to install the lithium ion secondary battery in an engine room of the vehicle in place of a lead storage battery.
Since an electrolytic solution of the lithium ion secondary battery is used at a high voltage of 2 V to 4.5 V, an aqueous solution-based electrolytic solution is not used in the lithium ion secondary battery, and a nonaqueous electrolytic solution in which lithium salt is dissolved in an organic solvent is used. It has been considered to improve a composition of the nonaqueous electrolytic solution in order to improve large current discharge performance and cycle life performance. However, since ion conductivity of the nonaqueous electrolytic solution is lower than that of the aqueous solution-based electrolytic solution, it is difficult to lower the resistance of a battery. Since an organic solvent is used in the nonaqueous electrolyte, high temperature decomposition of the nonaqueous electrolyte is likely to occur. And since heat stability of the nonaqueous electrolyte is poor, high-temperature cycle life performance is lowered. Also, although a solid electrolyte has been considered as a nonaqueous electrolyte, since the ion conductivity of the nonaqueous electrolyte is further lowered, it is difficult to enhance large current discharge performance.
In a nonaqueous electrolyte battery charged and discharged by movement of Li ions between a negative electrode and a positive electrode, a nonaqueous electrolyte containing a nonaqueous solvent is used as an electrolytic solution. Since the nonaqueous solvent has wide potential stability, in the nonaqueous electrolyte battery, a high cell voltage of approximately 3 to 4 V can be exhibited. Thus, the nonaqueous electrolyte battery is excellent in energy density as compared with conventional storage batteries. Therefore, in recent years, the use of nonaqueous electrolyte batteries has been progressing in a wide range of applications including on-vehicle application such as μHEV (micro-hybrid electric vehicle) and an idling stop system, and for stationary use.
However, since a nonaqueous solvent contained in a nonaqueous electrolyte is an organic solvent, the nonaqueous solvent is highly volatile and inflammable. Thus, a nonaqueous electrolyte battery has risks such as a possibility of ignition associated with over-charge, temperature increase, or impact. To prevent such risks, the use of an aqueous solvent in a lithium ion battery has been proposed.